doc: trace/kprobes: add information about NOKPROBE_SYMBOL

Update kprobe tracer documentation to also mention that
NOKPROBE_SYMBOL() and nokprobe_inline add symbols to the kprobes
blacklist.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/d924e20de099579ace4286e610304f054cd798db.1488991670.git.naveen.n.rao@linux.vnet.ibm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>